This application is based on Patent Application No. 2000-1046 filed on Jan. 6, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an encoder-decoder, and a CDMA (Code Division Multiple Access) communication system, especially, relates to a CDMA encoder-decoder and a CDMA communication system in an optical area wherein encoding and decoding of a signal can be carried out.
Also, this invention relates to a WDM (Wavelength Division Multiplexing)-CDMA communication system in an optical area wherein a CDMA technique is applied to a WDM communication.
2. Description of the Related Art
In an optical CDMA system, it is systematically possible to realize random access and self routing operation by encoding and decoding signals without using a component such as an optical switch. Therefore, this system can be applicable to an optical LAN and an optical switching system.
In the conventional art, such a configuration as shown in FIG. 20 has been used for an optical CDMA system.
In FIG. 20, optical pulse sources 1a-1c and a plurality of lattice-form optical circuits 2a-2f are opposed via a star coupler 3. Each lattice-form optical circuit includes cascaded as many as J (J: natural number) asymmetrical Mach-Zehnder interferometers in series where optical path length differences are xcex94L, 21xcex94L . . . , 2Jxe2x88x921xcex94L (J=2 in FIG. 20). Each lattice-form optical circuit positioned at the left and right side of the star coupler corresponds to an encoder and a decoder, respectively.
When the coupling coefficients of the directional couplers 4a-4f in the lattice-form optical circuits 2b and 2e are set at 0.5, and a short optical pulse with a repetition frequency of f(Hz){fxe2x89xa6c/(2Jnxcex94L), f=1/Tc, c: light speed in a vacuum, n: refractive index of a waveguide, Tc: pulse interval} and a pulse width of Tp enters the lattice-form optical circuit 2b, as many as 2J optical pulse trains {interval Tr(=nxcex94L/c)} are newly produced in a time frame of Tc(=1/f), thus code series are constructed.
These code series include phase information produced at refractive index control parts (phase shifters) 5a and 5b in the waveguide of the lattice-form optical circuit 2b. When this encoded optical pulse train enters the lattice-form optical circuit 2e, each optical pulse is separated into as many as 2J optical pulses and then electric field components of the pulses are coherently summed up.
When the settings of phase shifters 5c and 5d in the lattice-form optical circuit 2e satisfy the decoding conditions in contrast to phase shifters 5a and 5b, the optical power is concentrated in the center of each pulse to be decoded. However, when the settings do not satisfy the decoding conditions, an inputted encoded pulse is further spread in the time region and is not decoded.
FIGS. 21A to 21D shows the inputted pulse train (shown in FIG. 21A), the produced code series (shown in FIG. 2B) that have passed the encoder in the case of J=2, and outputs depending on the setting conditions of the decoder (FIG. 21C: when the decoding conditions are satisfied, FIG. 21D: when the decoding conditions are not satisfied). For simplicity, the effects produced by a single pulse in the inputted optical pulse train are shown.
FIG. 21B shows an example when the phases of the phase shifters in FIG. 20 are set that xcfx86a=xcfx86b=xcfx80, FIG. 21C is an example when xcfx86c=xcfx86d=0, and FIG. 21D is an example when xcfx86c=xcfx80 and xcfx86d=0. Note that xcfx86a, xcfx86b, xcfx86c, and xcfx86d are phase shift values of the phase shifters 5a, 5b, 5c, and 5d, respectively.
The conventional method described above, however, has a problem that side lobe components are produced around the decoded optical pulse, and the S/N ratio (Signal to Noise ratio) at the receiver is deteriorated even when the decoding condition is satisfied.
In addition, in the case of non-decoding, unnecessary optical pulses are further produced to deteriorate the S/N ratio of the signal. Namely, it indicates that both an auto-correlation and cross-correlation characteristics as an encoder and a decoder for the CDMA differ from the ideal condition.
Further, an encoded pattern can be decoded easily, thus causing a problem on the secrecy of communication that is one of the merits in the CDMA.
This is because the encoder is constructed so that every pulse having a same frequency component is multiplexed in the time region, and then the maximum value of multiplexing is limited and a spread factor (bit interval(Tc)/chip interval(Tr)) cannot be increased.
When an interval and a width of an optical pulse from an optical pulse source are denoted as Tc and Tp, respectively, the maximum spread factor is given as [Tc/Tp] ([R] provides an integer that does not exceed R, R: a real number). For example, this maximum rate is eight when Tc=25 psec and Tp=3 psec. As a result, the number of patterns, that encodes phases of a pulse to zero or xcfx80, is limited to 28(=256).
An object of the present invention is to provide a CDMA encoder-decoder and a CDMA communication system that can reduce the output levels of an unnecessary side lobe component of a decoded optical signal pulse and a non-decoded optical signal pulse by increasing the number of encoding patterns, that is; that can improve both an auto-correction and cross-correlation characteristics, and the secrecy of communication.
Another object of the invention is to provide a WDM-CDMA communication system that can decrease a wavelength channel interval in a WDM communication to improve the efficiency of frequency utilization by applying a CDMA encoding-decoding technique to the WDM communication.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.